The present invention relates to a cathode for electron tubes such as cathode-ray tubes (CRT) used for television or information displays.
As shown in FIG. 14, a conventional cathode for an electron tube includes a heater coil 101, a cylindrical sleeve 102 with the built-in heater coil 101, a metal substrate 103, containing nickel as a main component and a trace of reducing elements such as magnesium, at one opening of the sleeve 102, and an emissive material layer 104 adhered onto the substrate 103. For the emissive material layer 104, a material that includes as a main component an alkaline earth metal oxide containing barium is used as an oxide cathode. A phenomenon is found that the emission current of such a cathode gradually decreases after long operation of several thousand hours due to the deterioration of emissive materials.
Therefore, a proposal has been tested to improve the life of a cathode by adding from 0.3 wt. % to 15 wt. % of rare earth metals such as scandium oxide and yttrium oxide to an emissive material layer (Japanese Laid-open Patent Publication No. 62-22347).
Another proposal also has been tested whereby zirconium oxide or hafnium oxide is added to an emissive material layer at from 0.1 wt. % to 10 wt. % so as to extend the life of a cathode (Japanese Laid-open Patent Publication No. 2-195628).
Due to the recent increase in current density accompanied by the improvement of CRT display properties, there is a problem in that more and more load is added to a cathode, shortening the life of the cathode. Thus, a cathode has been demanded that has a longer life than conventional cathodes.